Which of these numbers is prime? ${5,\ 35,\ 75,\ 78,\ 91}$
Answer: A prime number has exactly two factors: 1 and itself. The factors of 5 are 1 and 5. The factors of 35 are 1, 5, 7, and 35. The factors of 75 are 1, 3, 5, 15, 25, and 75. The factors of 78 are 1, 2, 3, 6, 13, 26, 39, and 78. The factors of 91 are 1, 7, 13, and 91. Thus, 5 is a prime number.